Klystron having high frequency radiation means comprising a half-wave short-circuited choke



3,392,301 IATION MEANS UITED CHOKE July 9, 9 8 P. s. CAMPBEL KLYSTRON HAVING HIGH FREQUENCY COMPRISING A HALF-WAVE SHORT-CIRC Filed July 6, 1965 x m Jn x r W m 2 P MN m gm 2 2 2 w l N 4 A w K E m m 9 INVENTQR Awczzl ets United States Patent O 3,392,301 KLYSTRON HAVING HIGH FREQUENCY RADL' ATION MEANS COMPRISINGA HALF-WAVE SHORT-CIRCUITED CHOKE Peter Stanley Campbell, Rainham, Essex, England, as-

signor to English Electric Valve Company Limited, London, England, a British company Filed July 6, 1965, Ser. No. 469,652 Claims priority, application Great Britain, July 10, 1964,

28,580/64 6 Claims. (Cl. 3155.38)

ABSTRACT OF THE mscLosunn A high power klystron of the metal ceramic'construct tion type having a hollow collector which is spaced from the adjacent end of the last drift tube by a ceramic member which forms part of the evacuated envelope is disclosed. To prevent high frequency radiation from the gap between the last drift tube and the collector a half- Wave choke is provided which is external with respect to the gap and short circuited at its outer end outside the evacuated space of the klystron. The length of the halfwave choke is made up of the sum of the radial length of the gap, the length of a space communicating with the gap and situated between a part of the outer surface of the collector structure and a ceramic member which bridges the gap and forms part of the evacuated envelope of the tube, and the length, parallel to the axis of the tube, of the ceramic material of the ceramic member.

This invention relates to high frequency high power klystrons and more particularly to high frequency high power klystrons of so called metal-ceramic construction type wherein the electron beam, after passing the drift tubes of the klystron enters a 'hollow collector which is spaced from the adjacent end of the last drift tube by a ceramic member which form part of the evacuated envelope of the klystron.

It is of course necessary, in such a klystron, to prevent high frequency radiation escaping through the gap between the last drift tube and the collector for if this is not done the device will constitute, when operating, a serious source of high frequency interference with nearby other apparatus. The customary way of preventing such escaping radiation is, in effect, electrically to close the gap by associating therewith an open circuited quarter wave choke which is internally arranged within the evacuated envelope and is constituted by carefully dimensioned spaces between metal parts or metal coated parts of the structure in the neighbourhood of the gap. This type of construction calls for the adoption of very small clearances between the parts in question and also for the consequent adoption of undesirably close manufacturing dimension tolerances. Moreover, because of these very small clearances, relatively small distortion of parts due to thermal expansion or other causes may cause the choke to cease to be properly effective or, worse still, produce undesired short circuits between surfaces which are required to be clear of one another. The present invention seeks to avoid these difiiculties and defects. 1

According to this invention high frequency radiation from the gap between the last drift tube of a klystron and the collector thereof is prevented by means of a halfwave choke which is external with respect to said gap and is short circuited at its outer end outside the evacuated space of the klystron.

Preferably the length of the half wave choke is made up of the sum of the radial length of the gap, the length of a space communicating with said gap and situated between a part of the outer surface of the collector structure and a ceramic member which bridges the said gap and forms part of the evacuated envelope of the tube, and the length, parallel to the axis of the tube, of the ceramic material of said ceramic member. The said part of the outer surface of the collector structure is preferably (in the case of a water cooled collector) part of the outer wall of the cooling water jacket thereof.

Preferably also the ceramic member is metallised on 1ts inner and. outer surfaces, the metallisation on the inner surface being connected to a metal part of the klystron structure on one side only of the gap and the metallisation on the outer surface being connected to a metal part of the klystron structure on the other side only of said gap. In a preferred arrangement theinner metalhsation extends from one end of the ceramic member but stops short of theend thereof nearer the collector and the outer metallisation extends from the other end of said member but stops short of said one end.

.Preferably again the half wave choke is short circu ted at its outer end by means of a foil wrapping in electrical contact with the metallisation on the outer surface of the ceramic member and extending over but insulated as respects DC from a metal member in electrical connection with the last drift tube. The foil wrapping may be conveniently held in place by one or more encircling tensioned elastic rings.

The invention is illustrated in the accompanying drawing which shows schematically and half in section in the centre line, a preferred embodiment of the invention so far as is necessary to an understanding thereof. The drawing shows the adjacent ends of the last drift tube and the collector together with the parts adjacent the gap between said drift tube and collector. The remainder of the klystron is in accordance with known practice and is accordingly not illustrated.

Referring to the drawing 1 is the end of the last drift tube of the klystron and 2 is the hollow collector electrode thereof. The end of the collector nearer the drift tube is tapered inwardly in the customary way. 3 is a cap on the electron entry end of the collector which serves to prevent secondary electrons from the interiorof the collector travelling back to the drift tube and defines the entry aperture into the collector. The: collector is water jacketed for cooling the water jacket space, which is 1ndicated by the reference letter Wbeing the space between the parts 4 and 5 and the collector 2, containing a partition Wall 6 so that water circulation is up one side of this wall and down the other. It will be observed that the water jacket extends practically to the end of the col lector so that very effective cooling is obtained, the collector being water jacketed almost up to its entry end. The reference letter W indicates a water jacket space round the drift tube 1.

The said drift tube is seated in a mounting flange member 7 which is of ferro-magnetic material and forms part of the usual focussing magnetic circuit of the klystron. This flange is parallel to and spaced from the parts 3 and 4. A ceramic cylinder 8 is provided between the part 7 and a flange 9 extending outwardly from the outer wall of the collector water jacket. This ceramic cylinder is metallised by molybdenum which is fired on and then nickel plated, the metallisation M being on both inner and outer faces but stopping short of the end adjacent the flange 9 on the inner face and also stopping short of the end adjacent the flange 7 on the outer face. The ceramic cylinder 8 is fixed in position by means of re-entrant metal ring members 10 and 11 adjacent its ends as in the usual way. Each re-entrant ring is constituted by two rings welded together when they meet to give a vacuum seal the inner of these two rings being flanged and fitted to the ceramic as shown.

layer.12 of insulating tape,,preferably high voltage poly.-.

ester tape, the tape being also turned over the edge of the ring to the inside as shown. Over the tape is placed a wrapping 13 of metal foil, e.g. aluminium, which is insulated from the ring by the tape but is in good electrical contact with the metallisation M on the outside of the ceramic cylinder 8. Good contact between the foil 13 and the metallisation is maintained by an encircling tensioned neoprene rubber or similar elastic band 14. A wider similar outer rubber or like band is fitted, this band being of such Width as to extend over and between the rings 10 and 11. Thisholds the foil 13 tight against the tape 12 and provides an outer cover between the rings 10 and 11. The inner metallisation on the ceramic cylinder 8 is in good electrical contact with the ring 10 and the outer metallisation is in good electrical contact with the ring 11.

The gap between the parts 1 and 7 on the one hand and the parts 3 and 4 on the other; the space extending at right angles to that gap, and between the parts 5 and 8; and the dielectric-filled (ceramic-filled) space between the inner and outer metallic coatings M on the ceramic cylinder 8 are of such pre-determined combined length as to be electrically one half a wave length long at the mean designed working frequency of the tube so as to constitute a half wave length line which is short circuited at its outer end, outside the evacuated space of the tube, by means of the foil 13 which, as respects the working high frequency, short circuits the outer metallisation 0n the cylinder 8 to the upper ring 10, the tape 12, which is an insulator so far as DC is concerned being, of course, ineffective to prevent the required HF short circuit occurring. There is thus, in effect, a short-circuited half wave choke across the gap between the end of the drift tube 1 and the collector cap 3. This choke effectively closes the gap as respects radio frequency and prevents radiation and consequent interference with nearby apparatus. The construction is economical, convenient to manufacture and neat in appearance. Moreover, and more important, the radiation-preventing half-wave choke is not only external with respect to the gap from which radiation is to be prevented but extends outside the evacuated envelope of the tube, the outward part of the length of the choke being constituted by part of the envelope itself (the metallised ceramic cylinder 8) and being short circuited (by the foil 13) outside the vacuum. Accordingly the necessity for small clearances between parts and the accompanying necessity for close manufacturing dimension tolerances which arise in the usual known constructions in which radiation from the gap is prevented by means of open-ended quarter wave chokes arranged internally wholly inside the evacuated space, are avoided. Furthermore the liability, which is present with the usual known constructions referred to, of the occurrence of short circuits as a result of thermal expansion or distortion of parts, is also avoided.

I claim:

1. A high power klystron of the metal-ceramic construction type defining an evacuated envelope and including at least one drift tube and a hollow collector which is spaced from the adjacent end of the last drift tube by a ceramic member which forms part of the evacuated envelope wherein high frequency radiation from the gap between the last drift tubeand the collector is prevented by means of a half-wave choke which is external with'respe'ct to said gap and is short circuited at its outer end outside the evacuated space of the klystron, the length of the half wavec'hokebeing made up of the sum of the radial length of the gap, the length of a space communicating with said gap and situated between a part of the outer surface of the collector structure and a ceramic member which bridges said gap and forms part of the evacuated envelope of the tube, and the length, parallel to the axis of the tube, of the ceramic material of said ceramic member.

2. A klystron as claimed in claim 1 wherein the collector is water cooled and the said part of the outer surface of the collector structure is part of the outer wall of the cooling water jacket of the collector.

3. A klystron as claimed in claim 1 wherein the ceramic member is metallised on its inner and outer surfaces, the metallisation on the inner surface being connected to a metal part of the klystron structure on one side only of the gap and the metallisation on the outer surface being connected to a metal part of the klystron structure on the other side only of said gap.

4. A klystron as claimed in claim 3 wherein the inner metallisation of the ceramic member extends from one end of the ceramic member but stops short of the other end thereof nearer the collector and the outer metallisation extends from the other end of said member but stops short of said one end.

5. A klystron as claimed in claim 1 wherein the half wave choke is short circuited at its outer end by means of a foil wrapping in electrical contact with the metallisation on the outer surface of the ceramic member and extending .over but insulated as respects DC from a metal member in electrical connection with the last drift tube.

6. A klystron as claimed in claim 5 wherein the foil wrapping i held in place by one or more encircling tensioned elastic rings.

References Cited UNITED STATES PATENTS 2,678,392 5/ 1954 Blitz 33382 X 3,104,338 9/1963 Symons 3 l55.38 X 3,344,306 9/1967 Levin 315-538 X OTHER REFERENCES Montgomery et al. Principles of Microwave Circuits, McGraw-Hill, NY. 1948, p. 196 relied on.

a HERMAN KARL SAALBACH, Primary Examiner.

PAUL GENSLER, Examiner. 

